The explosion of the Internet and the desire to provide multiple communications and entertainment services to end users have created a need for a broadband network architecture that improves access to end users. One broadband network architecture that improves access to end users is a point-to-multipoint passive optical network (PON). A point-to-multipoint PON is an optical access network architecture that facilitates broadband communications between an optical line terminal (OLT) and multiple remote optical network units (ONUs) over a purely passive optical distribution network. A point-to-multipoint PON utilizes passive fiber optic splitters and couplers to passively distribute optical signals between the OLT and the remote ONUs.
FIGS. 1A and 1B represent the downstream and upstream flow of network traffic between an OLT 102 and three ONUs 104 in a point-to-multipoint PON. Although only three ONUs are depicted, more than three ONUs may be included in a point-to-multipoint PON. Referring to FIG. 1A, downstream traffic containing ONU-specific information blocks is transmitted from the OLT. The downstream traffic is optically split by a passive optical splitter 112 into three separate signals that each carries all of the ONU-specific information blocks. Each ONU reads the information blocks that are intended for the ONU and discards the information blocks that are intended for the other ONUs. For example, ONU-1 receives information blocks 1, 2, and 3, however it only delivers information block 1 to end user 1. Likewise, ONU-2 delivers information block 2 to end user 2 and ONU-3 delivers information block 3 to end user 3. Referring to FIG. 1B, upstream traffic is managed utilizing time division multiplexing, in which transmission time slots are dedicated to the ONUs. The time slots are synchronized so that upstream information blocks from the ONUs do not interfere with each other once the information blocks are coupled onto the common fiber 110, often referred to as the trunk. For example, ONU-1 transmits information block 1 in a first time slot, ONU-2 transmits information block 2 in a second non-overlapping time slot, and ONU-3 transmits information block 3 in a third non-overlapping time slot. As shown in FIG. 1B, all of the information blocks travel on the trunk in non-overlapping time slots.
Because point-to-multipoint PONs are intended to deliver integrated voice, data, and video services, existing point-to-multipoint PONs have been designed around the ATM data link protocol, which was designed with quality of service (QoS) features that enable integrated voice, data, and video delivery over a single communications channel. As is well known in the field of packet-switched communications, the ATM protocol transmits information in fixed-length 53 byte cells (48 bytes of payload and 5 bytes of overhead). In an ATM-based point-to-multipoint PON, fixed-length ATM cells are used to transmit information in both the downstream and upstream directions. For example, as disclosed in U.S. Pat. No. 5,978,374, each time slot in the upstream traffic flow is filled with a single fixed-length ATM cell and a fixed-length traffic control field.
Although the ATM protocol utilizes fixed-length 53-byte cells, ATM networks are often required to carry traffic that is formatted according to the widely used Internet protocol (IP). The Internet protocol calls for data to be segmented into variable-length datagrams of up to 65,535 bytes. In order for an ATM-based point-to-multipoint PON to carry IP traffic, the IP datagrams must be broken into 48 byte segments and a 5 byte header must be added. Breaking all incoming IP datagrams into 48 byte segments and adding a 5 byte header creates a large quantity of overhead that consumes valuable bandwidth in a point-to-multipoint PON. In addition to the increased bandwidth consumed by the ATM header, the process of converting IP datagrams into ATM cells is time consuming and the specialized hardware adds additional cost to the OLT and ONUs.
Another data link protocol that has been incorporated into a point-to-multipoint PON is the IEEE 802.3 protocol (commonly referred to as ethernet). Ethernet carries payload data (such as IP datagrams) in variable-length packets of up to 1,518 bytes. Although the ethernet protocol data units are described as “packets,” the protocol data units are also commonly referred to as ethernet “frames.” Using variable-length packets of up to 1,518 bytes in a point-to-multipoint PON can greatly reduce the overhead of IP traffic when compared to the overhead of an ATM-based point-to-multipoint PON. In addition to the advantage of reduced overhead, ethernet network components are relatively affordable.
When multiple stations in an ethernet network share a common physical transmission channel, the ethernet protocol utilizes a carrier sense multiple access/collision detection protocol (CSMA/CD) as a media access control mechanism to avoid collisions between transmitted traffic. CSMA/CD is an efficient media access control protocol that does not require multiple stations to be synchronized. Applying CSMA/CD in an ethernet network requires that the minimum length of a packet must be longer than the maximum round-trip propagation time of the network in order to avoid collisions that cannot be detected by all of the stations on the network. That is, the maximum distance of separation between users in a multi-station ethernet network is limited by the collision domain. For example, in an ethernet network operating at 1 Gb/s, the maximum separation between stations is limited by CSMA/CD to approximately 200 meters. For point-to-multipoint PONs to be commercially feasible, the OLT and ONUs need to be able to be separated by more than the maximum distance allowed by CSMA/CD. In addition to the collision domain limitation, ethernet networks that rely on CSMA/CD are non-deterministic. That is, QoS guarantees cannot be made for traffic between the OLT and the ONUs.
In view of the limitations of ATM-based point-to-multipoint PONs and ethernet-based point-to-multipoint PONs that utilize CSMA/CD, what is needed is a point-to-multipoint PON that utilizes variable-length packets and that increases the maximum allowable separation between the OLT and the ONUs.